An artist's impression made available by the European Southern Observatory shows a planet, right, orbiting the star Alpha Centauri B, centre, a member of the triple star system that is the closest to Earth. Alpha Centauri A is at left. The Earth's sun is visible at upper right. Photo: AP

This overheated world hugs the star Alpha Centauri B, zipping around it every three days. It is the nearest so-called exoplanet yet discovered.

The planet's Earth-like size and orbit around a sunlike star make it a landmark discovery, Stephane Udry of the University of Geneva, leader of the research team, said on Tuesday.

The new planet, dubbed Alpha Centauri Bb, is much closer to that star than Mercury is to our sun.

Alpha Centauri B is visible only from the southern hemisphere, so the research team studied it with instruments at the European Southern Observatory in Chile.

They detected the planet indirectly, by seeing Alpha Centauri B wobble at a speed of about 1.6km/h - a sign of a small planet tugging on it.

Detecting it was tricky, requiring 450 nights of observation over four years.

The new planet's Earth-like mass marks it as a rocky body, not a gas planet like Jupiter, said Xavier Dumusque, a University of Geneva astronomer and the lead author of a paper published online in the journal Nature describing the find.

Dumusque said it was likely that the planet's surface "is not solid but more like lava - like a 'lava planet' ".

One expert said the finding needed to be confirmed.

"Only if other analyses come to the same conclusion can we be sure that this planet exists," astronomer Artie Hatzes wrote in a companion article in Nature.

Udry, in response, said there was less than one chance in 1000 that the discovery was a phantom of the team's data.

As the closest stars to our sun, about four light-years distant, the Alpha Centauri system has long intrigued astronomers.

Unlike our solar system, the system contains three stars locked in a gravitational dance.

In the 1990s, astronomers listened to the Alpha Centauri system for alien radio broadcasts but heard nothing, said Seth Shostak, senior astronomer at the SETI (Search for Extraterrestrial Intelligence) Institute in California.

Any aliens on the new planet "would have to be devilish and enjoy hot weather", he said, adding that the SETI project would probably take another listen across a broader range of radio channels in case other, more habitable, planets also lurked in the system.

Since 1995, astronomers have listed 842 planets around other stars, according to one catalogue, revealing that most stars have planets.